Diesel fuel ranks second only to gasoline as a fuel for internal combustion engines. Trucks, buses, tractors, locomotives, ships, power generators, etc. are examples of devices that use diesel fuel. Passenger cars are another area of potential growth for the use of diesel engines that can provide improved fuel efficiency.
Unlike gasoline engines which operate by spark ignition, diesel engines employ compression ignition. In order to avoid long ignition delays resulting in rough engine operation, as well as to minimize misfiring and uneven or incomplete combustion which results in smoke in the exhaust gases that causes a major environmental problem, it is highly desirable to improve the burning quality of diesel fuels to minimize environmental pollutants such as hydrocarbons, carbo monoxide, particulate matters, etc. The cetane number (CN) is used to rate the ignition properties of diesel fuels. In general the cetane number depends primarily on its hydrocarbon composition. Saturated hydrocarbons, particularly those with straight, open chains, have relatively high cetane numbers, whereas unsaturated hydrocarbons have relatively low cetane numbers.
It is necessary to recognize that the relationship between the CN of diesel fuel and its performance cannot be equated in any way to the octane number of a gasoline and its performance in a spark-ignition engine. Raising the octane number allows an increase in the compression ratio and thus provides increased power and fuel economy at a particular fuel load. In contrast, in diesel engines, the desired CN provides good ignition at high loads and low atmospheric temperature. High cetane fuels eliminate engine roughness and diesel knock, allow engines to be started at lower temperatures, provide faster engine warm-up without misfiring or producing smoke and reduce formation of harmful deposits. On the other hand, too high cetane fuels can result in incomplete combustion and exhaust smoke due to too brief of an ignition delay which does not allow proper mixing of the fuel and air.
Commercial diesel fuels have CN numbers of at least 40. The suitable diesel fuel has appropriate volatility, pour and cloud point, viscosity, gravity, flash point and contain only small but tolerable levels of sulfur. It is also significant that carbon, residue formation and ash content should be kept low.
To enhance the properties of diesel fuels, particularly during ignition, cetane improvers are usually added. These compounds are typically aliphatic nitrates, such as isooctylnitrate. The stability, corrosiveness and toxicity of these or other multi-purpose additives are major issues. During winter in cold areas, ethanol is sometime added to diesel fuels to prevent fuel line and filter freezing. However, ethanol lowers the flash point of the fuel and increases corrosion problems.
It is known from U.S. Pat. No. 2,221,839 that straight chain aliphatic ethers such as n-butyl ether, n-amyl ether, mono-butyl ether of diethylene glycol, etc., can be used as ignition accelerators for hydrocarbon fuels of the compression ignition type. Generally, these ethers are added in an amount of as high as 50 or even 100% of the amount of the fuel. If desired, these ethers can be used by themselves as the fuel, although they are relatively more expensive.
In recent years, environmental concerns necessitate cleaner burning fuels with decreased detrimental emissions. Diesel fuels are no exception and there is need to diminish hydrocarbon, carbon monoxide, particulate matter, etc. emissions. Gasoline has been formulated with additives to help with this problem, but nothing useful has yet been developed for diesel fuel. Despite the need for similar additives for diesel fuel, no truly efficient diesel fuel improvers or enhancers have been discovered so far (See Gasoline and Motor Fuel, Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Wiley-Interscience, New York, Vol. 11, p.682-689, 1980).